The early detection of Trisomy 18, also known as Edwards Syndrome, is a significant focus in prenatal care due to the condition’s severity. This chromosomal disorder often leads to multiple structural malformations and a poor prognosis. Ultrasound imaging plays a central role in this process, providing the earliest visual cues that may indicate an increased risk. Understanding when and what an ultrasound can detect is essential for prenatal risk assessment.
Understanding Trisomy 18
Trisomy 18 is a genetic condition resulting from the presence of an extra copy of chromosome 18 in the body’s cells, meaning the individual has three copies instead of the usual two. This extra genetic material disrupts fetal development, leading to serious, life-limiting health issues affecting nearly every organ system. The vast majority of cases involve a complete trisomy, where every cell contains the extra chromosome 18. This condition occurs in approximately 1 in 5,000 to 6,000 live births.
The prognosis for a fetus diagnosed with Trisomy 18 is poor, with a high rate of miscarriage and stillbirth before term. For infants born alive, over 90% do not survive past their first year, often due to complications arising from severe congenital heart defects and respiratory failure. Establishing the presence of this condition early allows families and their medical team to prepare for potential outcomes and plan the course of care.
First Trimester Ultrasound Markers and Timing
The earliest opportunity for ultrasound to suggest an elevated risk for Trisomy 18 occurs during the first trimester, typically between 11 and 14 weeks of gestation. This initial screening scan focuses on measuring the Nuchal Translucency (NT), the fluid-filled space at the back of the developing baby’s neck. An abnormally thickened NT measurement is the most sensitive sonographic marker for Trisomy 18 during this early period, often being significantly larger than in unaffected pregnancies.
The detection rate for Trisomy 18 is high when this NT measurement is combined with maternal blood tests that analyze specific protein levels. Other early markers that may be visible on a specialized first-trimester ultrasound include the absence or underdevelopment of the nasal bone. Additionally, some major structural anomalies, such as an omphalocele, where abdominal organs protrude into the umbilical cord, can sometimes be identified. The presence of any of these findings raises the suspicion of a chromosomal abnormality, prompting discussion of further testing.
Second Trimester Ultrasound Findings
A more comprehensive assessment of the fetus occurs during the second trimester, typically between 18 and 20 weeks, through the detailed anatomy scan. By this time, many of the structural defects associated with Trisomy 18 have developed enough to be clearly visible on ultrasound, leading to a higher overall detection rate than the first-trimester screening. Nearly all fetuses with Trisomy 18 will exhibit one or more abnormal findings during this mid-pregnancy scan.
Major congenital heart defects are the most common and frequently detected anomaly, present in up to 90% of cases, often including ventricular septal defects (VSD) or atrial septal defects (ASD). Fetal growth restriction is another common finding, where the baby’s measurements fall below the fifth percentile for gestational age, often noted alongside a decreased amount of amniotic fluid.
Several other physical markers, sometimes called “soft markers,” can also be identified. These additional findings frequently include:
- A characteristic hand posture where the fingers are clenched and overlapping, often referred to as “clenched fists.”
- Central nervous system anomalies, such as the presence of choroid plexus cysts in the brain.
- “Rocker-bottom” feet.
- Kidney abnormalities.
- Diaphragmatic hernia.
The identification of multiple, unrelated structural anomalies across different organ systems during the second-trimester scan is highly suggestive of Trisomy 18.
Differentiating Screening from Definitive Diagnosis
Ultrasound findings from both the first and second trimesters are considered screening results; they indicate an increased risk but do not provide a definitive diagnosis of Trisomy 18. Ultrasound is a visualization tool that detects physical manifestations and markers, but it cannot analyze the chromosomes themselves. A definitive diagnosis requires genetic testing to confirm the presence of the extra chromosome 18.
When ultrasound or blood screening results suggest a high risk, invasive diagnostic procedures are offered to obtain a sample of fetal cells for karyotype analysis. Chorionic Villus Sampling (CVS) can be performed earlier, typically between 10 and 13 weeks of gestation, by taking a small sample of placental tissue. Alternatively, amniocentesis is performed later in the pregnancy, usually after 15 weeks, by sampling the amniotic fluid. Both procedures allow a laboratory to count the chromosomes and confirm the extra copy of chromosome 18, which is required for family counseling and medical planning.